This invention relates to a galvannealed electroplated steel strip having a ductile zinc/iron alloy coating and a process therefor. More particularly, a zinc electroplated strip is induction heated using low frequencies to interdiffuse zinc and iron to completely convert the zinc coating into an adherent zinc/iron alloy coating. It will be understood by a zinc coating is meant to include zinc and zinc base alloys. By a galvannealed strip is meant the formation of an alloy coating by heating the steel strip to an elevated temperature to allow interdiffusion of zinc from the zinc coating and iron from the base metal of the strip to form phases of zinc and iron other than those of the pure metals.
Converting a zinc coating to a zinc/iron alloy coating gives a steel strip a dull grey appearance rather than the shiny appearance of regular galvanized coating. The alloy coating has better abrasion resistance and a surface which is more suitable for painting. More importantly, increasing the iron content of the coating makes it much more weldable than regular galvanized strip. Accordingly, an iron rich coating or galvannealed steel strip is more acceptable in the automotive market.
It is well known to form a galvannealed steel strip by continuously hot dipping steel strip into a bath of molten zinc. The coating metal may be converted to a zinc/iron alloy coating by heating the zinc coated strip to an alloying temperature by radiant heating using direct fire burners placed adjacent to the strip or convection heating by heating the strip in a continuous furnace. It is also known to form a galvannealed strip by induction heating a continuously hot dip coated steel strip. Such an alloyed coating usually is given a conversion coating treatment by dipping in a zinc/iron phosphate solution and painted. It is difficult to obtain the necessary surface smoothness required for automotive exposed surfaces by galvannealing a hot dip coated strip.
Another disadvantage of forming a galvannealed strip using the continuous hot dip process is the high alloying temperatures required, e.g.; greater than 510.degree. C. Zinc coating baths contain a small amount of aluminum. The purpose of the aluminum addition is to retard a zinc/iron alloy formation when producing regular (non-alloyed) galvanized strip. The formation of a zinc/iron alloy layer at the interface between the steel substrate and zinc coating metal may result in poor coating metal adherence if the coated strip is fabricated into parts. Of course, a steel manufacturer generally cannot restrict an aluminum containing zinc coating metal to only regular galvanized strip. The manufacturer normally would have but a single galvanizing line and both type products, i.e., galvannealed and regular coated, would be produced on this hot dipping line.
From the zinc rich end of an iron/zinc equilibrium phase diagram, it is known four zinc alloy phases can form at galvanneal alloying temperatures. These phases are zeta (.zeta.) having about 7 atomic % iron, delta (.delta..sub.1) having about 8-13 atomic % iron, gamma one (.GAMMA..sub.1) having about 18-24 atomic % iron and gamma (.GAMMA.) having about 27-32 atomic % iron. For an alloyed coating, the amount of the .zeta. phase is probably insignificant since its stability range is narrow. Of the three remaining phases, the .delta..sub.1 phase is very desirable because it is more ductile than the .GAMMA. and .GAMMA..sub.1 phases. The diffusion process proceeds with iron migrating from the surface of the steel strip toward the outer surface of the zinc coating. An iron concentration gradient exists through the zinc coating thickness. Since the zinc coating must be completely alloyed to its outermost surface so that the coating can be welded and painted, it becomes extremely difficult to eliminate or minimize the formation of the brittle .GAMMA. and .GAMMA..sub.1 phases at the surface of the steel strip when using long times and/or high annealing temperatures required for galvannealed continuously hot dip coated steep strip.
It has been previously proposed a galvannealed strip can be produced by induction heating a zinc electroplated strip. Japanese published application 59/9163 discloses alloying a one-side zinc electroplated strip by high frequency induction heating. This Japanese application suggests the surface of a zinc coated steel strip can be heated by high frequencies, which provides an improvement in operation control, and the resulting quality is comparable to a product produced with radiant heating using a direct fired furnace.
Magnetic materials such as ferritic carbon steel also can be heated at low frequencies by inducing eddy current into the steel through the action of an external alternating magnetic field. High frequencies, otherwise known as radio frequencies, are generally defined as about 10 kHz to over 27 MHz. Induced eddy currents produced using radio frequencies are concentrated at the surface of the material with the depth of current penetration determined by the magnetic and electrical properties of the steel. This depth or thickness of the so-called "skin effect" can be calculated by the formula d=5000(p/.mu.f).sup.1/2 where d is the reference depth (cm), p is the specific electrical (or "volume") resistivity of the heated material (ohm-cm), .mu. is the relative permeability and f is the frequency of the applied external magnetic field. Of these properties, the permeability will remain relatively unchanged during the heating process. However, the specific resistance increases with temperature by about 0.125 uohm-cm/.degree.C. At a frequency of 100 kHz, the reference depth for a magnetic carbon steel has been determined to be 0.003 cm at about 150.degree. C. and increasing to only 0.006 cm at about 700.degree. C. When the frequency is reduced to low levels, i.e., not greater than 10 kHz, the current penetrates into the steel. Unlike high frequency heating which heats only the surface or skin of the steel, low frequencies heat the steel uniformly and rather homogeneously. The most efficient heating condition is at a low frequency wherein the current penetration depth is one-half the thickness of the material.
Accordingly, there remains a long felt need for an economical process for producing galvannealed strip wherein the coating metal is completely alloyed with iron and the iron concentration is controlled so that the resulting zinc/iron alloy coating is strongly adherent to the steel substrate and will not crack or craze when the steel strip is fabricated. Furthermore, there remains a need for such an alloy coating that provides good conversion coating and an excellent substrate for automotive paint finishing systems.